Vascular bypass grafts often fail when neointimal fibromuscular hyperplasia (NFH) causes stenosis and flow reduction. The usual explanation of NFH is that platelets adhere to grafts and release a platelet-derived growth factor (PDGF) which is mitogenic and chemotactic for vascular smooth muscle cells (SMC). But this "platelet-based" theory is weakened by evidence that NFH can occur (a) despite antiplatelet drug use and (b) in areas covered by endothelium which prevents platelets adhesion. The discovery that endothelial cells (EC) can make SMC mitogens suggests a different hypotheses: That EC are (i) normally in a nonproliferating state on normal basement membranes with little or no SMC mitogen release, but (ii) undergo a phenotype change to high SMC mitogen synthesis and release when stimulated to proliferate on fibrin thrombi or fibronectin coated grafts during a wound healing response, and (iii) that the SMC mitogens made by damaged or proliferating EC on grafts or host vessels act on subjacent SMC to cause NFH. We will do four kinds of experiments to test this hypothesis: (1) We will study how SMC mitogen gene expression and release from EC are regulated by (a) extracellular matrix components (fibronectin, laminin, and basement-membrane-like complexes) and (b) by growth factors and heparin used to grow adult human EC for graft linings. These effects will be measured by dot blots for PDGF gene transcription, mitogen release assays of conditioned medium, and flow cytometry of DNA levels. (2) To see if increased SMC mitogen gene transcription occurs in vivo at sites of NFH, we will use in situ hybridization of radioactive labeled cDNA probes for SMC mitogen mRNA sequences to study NFH lesions from stenosed coronary bypass grafts. This will allow histologic detection of SMC mitogen gene expression in EC or SMC at NFH sites. (3) To see if EC-derived SMC mitogens can act directly on subjacent SMC layers, we will cocultivate EC and SMC layers in direct contact in dishes and tubular graft models and use flow cytometry to measure the mitogenic response of the SMC to controlled changes in endothelial production of SMC mitogens. (4) If we find that SMC mitogens can cause SMC hyperplasia in such models and are transcribed in vivo in NFH lesions, we will try to construct graft surfaces which mimic normal basement membrane and which can make EC revert to a quiescent phenotype where SMC mitogen production is reduced or absent and EC-platelet or EC-white cell interaction would be minimized on EC-seeded grafts with such linings.